A solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the backside. When solar radiation of an appropriate wavelength falls on a p-n junction of a semiconductor body, hole-electron pairs are generated in that body. Holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current through the contacts disposed on the front-side and backside, which is capable of delivering power to an external circuit. The contact on the front side is generally made in the form of a grid comprising widely-spaced thin metal lines, or fingers, that supply current to a larger bus bar. The back contact is generally not constrained to be formed in multiple thin metal lines, since it does not prevent incident light from striking solar cell.
In order to increase the power generation characteristics of the solar cell the characteristic of the conversion efficiency EFF (%) is particularly important factor among the factors determining the efficiency of a solar cell. In order to increase the conversion efficiency, suitable electrode (grid) design is such as to attain high conduciveness, to cause electrons to move efficiently and to not decrease the area of the front surface on which solar radiation is incident. As a result; the power generation efficiency is increased. In order to achieve this objective, a variety of solar cell manufacturing techniques for fabricating electrodes having a high-aspect ratio for a solar cell by applying a conductive paste having a predetermined viscosity range (for example, by screen-printing) and for attaining superior conversion efficiency EFF (%) have been proposed.
A process for forming solar cell electrodes having a high aspect ratio, which attains superior conversion efficiency EFF (%) by screen-printing a conductive paste containing carbon fibers have been disclosed in US-2010-0294353 A1. The process described therein involves the screen printing and there are limitations to the aspect ratio of the formed electrode by screen printing. Moreover, the aspect ratio of the formed electrode is decreased after the firing process.
Japanese published patent application No. 2011-5404 (Kokai) describes a printing method and a device for forming solar cell electrodes by applying conductive paste through the discharge slot of a nozzle dispenser onto the wafer. Despite the numerous techniques utilized, a need still exists for electrodes for solar cells having superior conversion efficiency EFF (%), while increasing the requirement for reducing damage to global environment and cost reduction.
Solar cell electrodes are composed of collector electrodes (finger electrodes) and connector electrodes (bus electrodes). It is important that the finger electrodes be formed with a narrow line width so that more light can enter the front surface of the wafer. Thus, there is a need for a conductive paste capable of forming fine lines with a high aspect ratio. For the bus electrodes, particularly important features include high conductivity, tab connections, and adhesive strength.
The functions required for the finger electrodes and the bus electrodes are different. However, it has been a common practice to form both the finger electrodes and the bus electrodes with the same conductive paste suited to forming the finger electrodes.